All high speed receivers generally experience some kind of spurious responses. These responses are often caused by undesired, but reasonably predictable, contamination signals riding on the radio frequency (RF) or local oscillator (LO) signals that convert unintentionally to the intermediate frequency (IF) signal, and hence get recognized as a desired response.
These contamination signals can arise from many sources, e.g., reference clocks, digital signaling frequencies, component frequencies used in the creation of the RF or LO signal and strong external interferers. Sometimes it is possible to avoid these spurious responses by dynamically changing the configuration of the receiver, such as by changing the constituent makeup of the RF or LO signal. The problem of spurious responses becomes more apparent in the case of a fast measuring receiver where speed is critical and even small spurious responses can be problematic. Some conventional techniques to reduce the effect of spurious responses with a high speed receiver are described with reference to FIGS. 1-3.
FIG. 1 is an example of a typical receiver 100. The receiver 100 includes a group of conversion stages 1021 through 102n. A common configuration, however, uses one conversion stage, or n=1, for both speed and cost reasons. In FIG. 1, the conversion stages 1021-102n can include local oscillators 1181-118n that mix with a RF input signal 104 to convert the RF signal 104 into IF signals 1061 through 106n. The conversion stages 1021-102n are each shown as a mixer but can be a harmonic sampler or other structure that converts the RF 104 signal into signals IF1-IFn. The IF1 signal 1061 is provided through a bandpass filter 112 into the second conversion stage. After the final conversion stage 102n, the IFn signal 106n is provided through a bandpass filter 114 into analog-to-digital (A/D) converter 116.
In an exemplary embodiment that illustrates how a spurious response arises with the circuitry of FIG. 1, consider a single conversion stage (n=1) where the IF signal is 20 MHz, the RF signal is 20 GHz and the LO is set at 20.02 GHz. Consider further that the LO is contaminated with 4 MHz spurious response from a digital clock and has sidebands spaced at 10 MHz from the carrier (for 4 harmonics in each direction) due to the frequency reference of a synthesizer. Suppose also that the LO is generated by combining a multiple of a 4.995 GHz oscillator and a direct digital synthesizer (DDS) set at 40 MHz. A spurious response can arise for several reasons. First, the 5th harmonic of the 4 MHz digital clock can land right on the IF signal. Second, the 20 MHz offset sideband on the LO can self-mix with the LO to produce the IF signal. Third, the DDS leakage can mix with the harmonics of the digital clock or downconverted versions of the LO sidebands to produce the IF signal. Fourth, multiples of the 4.995 GHz leakage can mix with the RF signal to produce the undesired IF signal. The spurious generation of the IF signal can overwhelm the desired signal, particularly if the RF signal is low in amplitude.
In the past, several techniques using multiple conversion stages have been employed to avoid spurious responses. One reason multiple conversion structures are used to avoid spurious responses is because the intermediate filtering performed at each conversion stage can sometimes be used to reduce spurious responses. For example, see U.S. Pat. No. 6,785,527, entitled “Conversion spur avoidance in a multi-conversion radio frequency receiver” (Earls), filed Apr. 2, 2001, and U.S. Pat. No. 5,640,697, entitled “Wideband multiple conversion receiver system with means for avoiding receiver spurs by combined switching of multiple local oscillator frequencies” (Orndorff), filed Jun. 7, 1995. Systems of both these patents dynamically modify the first IF signal in a two-conversion system to avoid specific problems. However, often the final conversion to an IF signal is not changed since either a) the final IF signal is a conversion to direct current (DC), or b) changing the final IF signal would require A/D clocking changes, and hence, problems associated with clocking the output digital data stream. Thus, the previous IF signal is changed as needed to avoid spurs.
In order to effectively change an IF signal to avoid spurious responses, an analysis similar to that performed in section II.C of A. Hietala, et al., entitled “Self-shielded quad-band EGPRS transceiver with spur avoidance”, IEEE Trans. Micr. Theory Tech., Vol. 57, April 2009, pp. 910-918; can be performed to determine where spurious responses occur. The Hietala system is a two conversion system, with the first conversion mode being dynamic. The final IF signal is at DC so the last conversion stage is not altered. The possible mix of products of the various signals in Hietala and the conditions of a spurious response are often expressed as the following (where k signals are considered):fIF−Δ≦n1·f1±n2·f2± . . . ±nkfk≦fIF+Δ,
where if only integer multiples are considered, then the ni are integers representing possible spurious responses. In some cases, the ni may only be rational numbers. Since a larger ni tends to correspond to lower amplitude spurious responses (mixing processes get much less efficient at higher orders, usually at the rate of log10(ni)), there will usually be an upper limit on the size of ni that is a concern. The parameter Δ describes the range of concern and is normally related to the bandwidth of the final IF system.
When a spurious response that is of concern is identified, the fIF may be moved, or some of the constituent signals may be changed to avoid the spurious response. To better understand how the constituent oscillator changes can help, consider the exemplary embodiment described above. Instead of combining a multiple of a 4.995 GHz synthesizer and a 40 MHz DDS, one might use a 4.997 GHz synthesizer setting and a 32 MHz DDS setting. This configuration will cause fewer issues with the given IF signal since some of the constituent signals have been changed.
A disadvantage to making slight adjustments to fIF at individual frequencies to remove a spurious response, particularly for measuring receivers, is that there are varying levels of concern about the removed spurious response depending on how the measurement is setup. For example, when operating the receiver at very high speeds over wide bandwidths, the range of concern increases (due to the wide bandwidth) but the level of concern decreases since the noise floor will be higher, thus masking some low level spurs. If operating at low speed over narrow bandwidths, the range of concern decreases since bandwidth is not wide while the level of concern increases since spurs are now more pronounced. The traditional approach of adjusting fIF to remove a spurious response ignores how the measurement is setup, and hence may try to avoid too many spurious responses (thus slowing down the operation), or avoid too few spurious responses.
Returning to the use of multiple conversion receivers, FIG. 2 illustrates an alternative approach to avoiding spurious responses. In FIG. 2, an input RF signal 208 is passed to receivers 2161-216q. The receivers 2161-216q respectively each include bandpass filters 2221-222q, LOs 2021-202q and A/D converters 2281-228q. Each receiver has a different frequency plan as a result of the different components in each receiver. Due to the different frequency plan, each receiver will produce a different spurious response. As such, at any given input RF signal 208, the best receiver (in the sense of the least amount of spurious response on IF signals 2101-210q) can be chosen. However, similar to the approach discussed in FIG. 1, a disadvantage is that this technique is costly and complex.
FIG. 3 illustrates a related approach to FIGS. 1-2. As illustrated in FIG. 3, there is a single conversion stage 302 but multiple A/D stages (stages 3041-304q). The input RF signal 310 is converted at conversion stage 302 into IF signals 3121-312q. Each IF signal is passed to respective A/D stage 3041-304q. The IF signal at each A/D stage is then passed through respective bandpass filter 3181-318q to A/D converter 3241-324q. By using multiple A/Ds, the digital data stream complexity is reduced at the expense of complex clocking systems and parallel processing paths. However, because of all the components, this approach is still costly and complex.
Another approach that is sometimes used to avoid spurious responses, particularly at lower frequencies, is direct digital acquisition. That is, the RF signal is directly sampled by an A/D converter and the results digitally filtered. However, a disadvantage to this approach is that it has limited bandwidth (or is quite expensive if bandwidth is not limited), can be slow and can be sensitive to strong interferers that saturate the A/D converter.